Understanding the structure and properties of geological formations can reduce the cost of drilling wells for oil and gas exploration. Measurements made in a borehole (i.e., downhole measurements) are typically performed to attain this understanding, to identify the composition and distribution of material that surrounds the measurement device downhole. To obtain such measurements, pulsed neutron sources are often used to measure naturally-occurring gamma radiation downhole. Pulsed neutron spectroscopy involves bombarding a formation with high-energy neutrons and measuring the spectrum of gamma rays emitted by the formation. Different nuclei in formation materials emit characteristic gamma rays, allowing for the estimation of formation constituents and properties such as porosity and oil/gas saturation.
Saturation calculation in some types of formations, such as shaly formations, using pulsed neutron instruments has always been challenging because of the issue of shale handling. For example, pulsed neutron log analysis in shaly sands is complicated because minor variations in shale volume can have a large impact on measurements. Such variations can thus significantly compromise the accuracy of pulsed neutron measurements and the corresponding analysis. Ongoing efforts are directed to improving accuracy of pulsed neutron measurements in formations that include shale.